Fluorescent dyes that exhibit an enhanced fluorescence upon binding to DNA or RNA molecules are a basic tool in molecular and cell biology. Majority of the existing dyes bind noncovalently to nucleic acids through a combination of hydrophobic interactions with the DNA base-pairs and ionic binding to the negatively charged phosphate backbone. The most useful dyes are preferably non-fluorescent in the absence of the nucleic acids. Upon binding to DNA or RNA, however, several-fold fluorescence enhancement is measured, thereby enabling the detection of small amounts of nucleic acids.
A variety of fluorescent dyes have been shown to be effective stains for nucleic acids. Examples of fluorescent DNA-binding dyes include ethidium bromide, SYBR Green and SYBR Gold, which are commonly used to stain DNA in agarose gels, Hoechst 33258 and propidium iodide useful in flow cytometry or cell-impermeant TOTO dyes frequently used as a convenient indicator of apoptosis. Selected properties of these dyes such as the brightness, spectral properties, resistance to photobleaching, affinity for either single or double stranded nucleic acids can be modified to some extend with changing the type and location of the substituents.
In order for a fluorescent dye to be a useful tool in molecular and cell biology, the dye has to meet several requirements. First, it should preferably have an excitation maximum between 300 and 900 nm, with a Stoke shift of at least 10 nm. The most preferable dyes have an excitation maximum between 500 and 900 nm. This spectroscopic range is both compatible with the existing detection instrumentation and removed from any other interfering biological molecules. Fluorescent dyes used in nucleic acid research should also have a high molar extinction coefficient, a high quantum yield, good photostability, and significantly increased fluorescence when bound to nucleic acids.
In U.S. Pat. No. 4,937,198, a purine-based fluorescent dye was disclosed that selectively stained nucleic acids. The dye was reported to have a molar extinction coefficient of 60,000 M−1 cm−1, quantum yield of 0.4 and excitation maximum in the presence of nucleic acids around 460 nm with an emission maximum around 478 nm. Fluorescence enhancement of over 7,000-fold was measured upon binding of the dye to RNA.
Moreda and Forrester (Tetrahedron, 53, 12595, 1997 and Tetrahedron, 53, 12605, 1997) described a synthesis and characterization of a range of purine-based fluorescent dyes that were used as fluorescent dyes for DNA marking in the diagnosis of malaria parasites.